1. Field of the Invention
The present invention relates to a dielectric filter and a antenna duplexer using the same.
2. Description of the Related Art
Conventionally, a dielectric filter using a dielectric coaxial resonator has been constructed by respectively connecting one-end face short-circuited type coaxial resonators each formed by providing a dielectric member with a through hole and coating an outer peripheral surface of the dielectric member and an inner peripheral surface of the through hole with a conductive member (for example, silver) and capacitive-coupling electrodes formed on the dielectric substrate to the respective coaxial resonators.
In recent years, communication apparatuses have been decreased in weight and bulk in the field of mobile communication. With decreasing weight and bulk, smaller-sized dielectric filters have been requested.
Meanwhile, the ratio of an inner coaxial diameter to an outer coaxial diameter must be 3.6 so as to obtain a high Qu value (unloaded Q value). In manufacturing a small-sized dielectric filter, therefore, if the outer coaxial diameter is not more than 4 mm, the inner coaxial diameter is not more than 1.2 mm. It is thus difficult to insert a member for external connection into the through holes of the above described coaxial resonators to connect the same with an external circuit as described above. The present applicant has proposed U.S. patent application Ser. No. 671,615 as a dielectric filter for solving the problem.
Furthermore, when a filter is formed by coaxial resonators comprising not less than three stages, the length of the coaxial resonator in the middle stage is made longer than those of the coaxial resonators in the first stage and the final stage. Therefore, there occurs the problem that coaxial resonators having various lengths are required.
On the other hand, in the mobile communication apparatus, a antenna duplexer for separating and combining signals having different frequencies depending on the frequency is used. Such an antenna duplexer comprises a dielectric filter for transmission and a dielectric filter for reception which differ in center frequency. In such a dielectric filter, the interval between the center frequencies of a receiving band and a transmitting band becomes shorter with higher frequencies in mobile communication, so that it is difficult to obtain required out-of-passband attenuation. Therefore, the dielectric filter used in the antenna duplexer must have an attenuation pole in its characteristics.
Examples of a method of forming a pole in an attenuation region include one for directly connecting resonators by a reactance element with at least one resonator being skipped to form such a pole in a dielectric filter disclosed in Japanese Patent Laid-Open Gazette No. 77703/1987.
In this construction, however, the number of components is increased, and the assembly becomes complicated.
In order to solve the problem, a dielectric filter as shown in FIGS. 31 and 34 in Japanese Patent Application No. 46796/1991 has been proposed. Description is made of the dielectric filter with reference to the drawings. In this dielectric filter, five coaxial resonators 1.sub.1 to 1.sub.5 constitute a filter. Each of the resonators 1.sub.1 to 1.sub.5 is provided with a depressed part 2. A dielectric substrate 3 is mounted on the depressed part 2. The resonators 1.sub.1 to 1.sub.5 are coupled to each other through coupling windows.
Input-output connection electrode 3a' and 3a" and capacitance forming electrodes 3c', 3c" and 3c'" are formed on the upper surface of the dielectric substrate 3', while stab electrodes for external connection 3d' and 3d" formed by extending the input-output connection electrodes 3a' and 3a" and a ground electrode 3b are formed on the lower surface thereof.
Input-output capacitances C1 and C'1 are respectively formed between the input-output connection electrodes 3a' and 3a" and the resonators 1.sub.1 and 1.sub.5. In addition, resonator length correcting capacitances C2, C'2 and C"2 are respectively formed between the capacitance forming electrodes 3c, 3c" and 3c'" and inner peripheral conductors 1d.sub.2, 1d.sub.3 and 1d.sub.4 of the coaxial resonators 1.sub.2, 1.sub.3 and 1.sub.4. Furthermore, jump coupling capacitances C3, C3' and C3" are respectively formed between the input-output connection electrodes 3a' and 3a" and the capacitance forming electrodes 3c', 3c" and 3c'".
As shown in FIGS. 33 and 34, in the dielectric filter, a pole P is formed in an attenuation region of filter characteristics by the jump coupling capacitances C3, C'3 and C"3.
In the above described dielectric filter, however, when the grounding of the ground electrode 3b on the dielectric substrate 3 is incomplete, a capacitive coupling is achieved between the connection electrodes 3d' and 3d" through the ground electrode 3b, significantly degrading the filter characteristics. It is difficult to obtain complete grounding particularly in a microwave band of not less than 1 GHz, which is a serious problem.
More specifically, it is actually difficult to obtain complete grounding, so that an impedance component is contained between the ground and the ground electrode 3b. In this case, a part, which is not coupled to the resonators, of power from the input side is directly coupled to the output side through a capacitance. In this case, the out-of-band attenuation is significantly degraded in the frequency characteristics of the filter.
In order to prevent the degradation, the ground electrode 3b may be removed from the dielectric substrate 3. However, there is a possibility that noise from an external circuit or element in close proximity to the filter is coupled to inner peripheral conductors 1d.sub.1 and 1d.sub.5 of the resonators 1.sub.1 to 1.sub.5 or the connection electrodes 3d' and 3d" on the dielectric substrate 3, resulting in degradation of the filter characteristics.